A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H2SO4

Phosphorous as a minor element (similar to 0.03%) in AISI 304 austenitic stainless steel greatly affects the polarization and corrosion behavior in sulfuric acid solution. The presence of P in stainless steel created a second current maximum in the anodic polarization curve and the current increased with increasing aging in the solution. An adhesive corrosion surface layer, rich with phosphate, formed on the surface of a P-containing steel during active dissolution. The layer lowered the cathodic Tafel slope at low current densities, and is likely, due to a change in hydrogen evolution mechanism. Phosphorous increases the H-adsorbed (and/or absorbed) atoms on the surface, leading to the appearance of a second anodic current peak that is interpreted as re-oxidation of hydrogen atoms. Also, P shifted the corrosion potential to the noble side, decreased effectively the active anodic dissolution, and lowered the corrosion rate. (C) 1999 Elsevier Science Ltd. All rights reserved.